For the production of an alpha-olefin polymer, particularly an ethylene polymer or an ethylene/alpha-olefin copolymer, methods have previously been known in which ethylene is polymerized, or ethylene and an alpha-olefin are copolymerized, in the presence of a titanium-based catalyst comprising a titanium compound and an organoaluminum compound or a vanadium-based catalyst comprising a vanadium compound and an organoaluminum compound.
On the other hand, catalysts comprising a zirconium compound and an aluminoxane have been proposed recently as a new type of Ziegler catalyst for olefin polymerization.
Japanese Laid-Open Patent Publication No. 19309/1983 describes a process which comprises polymerizing ethylene and at least one alpha-olefin having 3 to 12 carbon atoms at a temperature of -50.degree. to 200.degree. C. in the presence of a catalyst composed of a transition metal-containing catalyst represented by the following formula EQU (cyclopentadienyl).sub.2 MeRHal
wherein R represents cyclopentadienyl, C.sub.1 -C.sub.6 alkyl or halogen, Me represents a transition metal, and Hal represents halogen, and a linear aluminoxane represented by the following formula EQU Al.sub.2 OR.sub.4 (Al(R)--O).sub.n
wherein R represents methyl or ethyl, and n is the number of 4 to 20, or a cyclic aluminoxane represented by the following formula ##STR1## wherein R and n have the same definitions as given above.
This patent document states that in order to adjust the density of the resulting polyethylene, ethylene should be polymerized in the presence of a small amount (up to 10% by weight) of a slightly long-chain alpha-olefin or a mixture thereof.
Japanese Laid-Open Patent Publication No. 95292/1984 describes an invention relating to a process for producing a linear aluminoxane represented by the following formula ##STR2## wherein n is 2 to 40 and R is C.sub.1 -C.sub.6 alkyl, and a cyclic aluminoxane represented by the following formula ##STR3## wherein R and n have the same definitions as given above.
This publication states that when an olefin is polymerized using a mixture of, for example, methylaluminoxane produced by the above process with a bis(cyclopentadienyl) compound of titanium or zirconium, a polyethylene is obtained in an amount of at least 25 million grams per gram of the transition metal per hour.
Japanese Laid-Open Patent Publication No. 35005/1985 discloses a process for producing a catalyst for polymerization of olefins, which comprises reacting an aluminoxane compound represented by the following formula ##STR4## wherein R.sup.1 represents C.sub.1 -C.sub.10 alkyl, and R.sup.0 is R.sup.1 or is bonded to represent --O--, with a magnesium compound, then chlorinating the reaction product, and treating the chlorinated product with a compound of Ti, V, Zr or Cr. The above publication describes that the above catalyst is especially suitable for the copolymerization of ethylene with a C.sub.3 -C.sub.12 alpha-olefin mixture.
Japanese Laid-Open Patent Publication No. 35006/1985 discloses a combination of (a) at least two different compounds selected from a mono-, di- or tri-cyclopentadienyl compound of transition metals or its derivative with (b) an alumoxane (an aluminoxane) as a catalyst system for blending polymers in a reactor. Example 1 of this publication discloses that a polyethylene having a number average molecular weight of 15,300 and a weight average molecular weight of 36,400 and containing 3.4 mole % of a propylene component was obtained by polymerizing ethylene and propylene using bis(pentamethylcyclopentadienyl)dimethylzirconium and an alumoxane as a catalyst. In Example 2 of this publication, a blend of a polyethylene and an ethylene/propylene copolymer having a number average molecular weight of 2,000, a weight average molecular weight of 8,300 and a propylene component content of 7.1 mole % and consisting of a toluene-soluble portion having a number average molecular weight of 2,200, a weight average molecular weight of 11,900 and a propylene component content of 30 mole % and a toluene-insoluble portion having a number average molecular weight of 3,000, a weight average molecular weight of 7,400 and a propylene component content of 4.8 mole % was obtained by polymerizing ethylene and propylene using bis(pentamethylcyclopentadienyl)zirconium dichloride, bis(methylcyclopentadienyl)zirconium dichloride and an alumoxane as a catalyst. Likewise, Example 3 of this publication describes a blend of a LLDPE and an ethylene/propylene copolymer composed of a soluble portion having a molecular weight distribution (Mw/Mn) of 4.57 and an propylene content of 20.6 mole % and an insoluble portion having a molecular weight distribution of 3.04 and a propylene component content of 2.9 mole %.
Japanese Laid-Open Patent Publication No. 35007/1985 describes a process which comprises polymerizing ethylene alone or with an alpha-olefin having at least 3 carbon atoms in the presence of a catalyst system comprising a metallocene and a linear alumoxane represented bu the following formula ##STR5## wherein R represents an alkyl group having 1 to 5 carbon atoms and n has the same definition as given above.
The publication describes that the polymer obtained by the above process has a weight average molecular weight of about 500 to about 1,400,000 and a molecular weight distribution of 1.5 to 4.0.
Japanese Laid-Open Patent Publication No. 35008/1985 describes that a polyethylene or a copolymer of ethylene and a C.sub.3 -C.sub.10 alpha-olefin having a wide molecular weight distribution is produced by using a catalyst system comprising at least two types of metallocenes and an alumoxane. The publication states that the above copolymer has a molecular weight distribution (Mw/Mn) of 2 to 50.
Furthermore, Japanese Laid-Open Patent Publications Nos. 260602/1985 and 130604/1985 propose a process for polymerizing olefins using catalysts formed from a transition metal compound and a mixed organoaluminum compound composed of an aluminoxane and an organoaluminum compound. The publications state that the addition of the organoaluminum compound gives increased polymerization activity.